The present invention relates to fabrication of microelectronic circuit components, interconnections and packages, and to articles useful in such processes.
Many microelectronic assemblies employ panel-like circuit elements. For example, one common method of connecting the contacts on a semiconductor chip to external circuitry, referred to as tape automated bonding or xe2x80x9cTABxe2x80x9d uses a sheet-like tape including a flexible dielectric layer, typically polyimide with metallic circuit traces thereon. Ordinarily, these circuit traces are formed by photochemical processes such as etching or plating using photographically patterned resists. The precision with which such a circuit can be formed is limited by the dimensional stability of the dielectric layer during processing. This problem increases as the size of the circuit increases. Typical TAB tape has numerous individual circuits made by photographically patterning an area of the flexible circuit of the size required to mount a single chip. The individual circuits are spaced along the length of the chip. Because the process only requires registration of features over a relatively small region corresponding to the dimensions of an individual chip, there is no need to maintain precise spacing between widely separated features. Moreover, typical TAB tapes do not require especially precise registration between features formed in different stages of the manufacturing process.
Larger circuits which require precise relative location of widely-spaced features have been fabricated heretofore using a xe2x80x9cdecalxe2x80x9d or xe2x80x9cappliquexe2x80x9d approach in which the flexible circuit is fabricated on the surface of a metallic plate. The metallic plate is then removed, as by exposure to a caustic etching process. For example, certain embodiments of U.S. Pat. No. 5,055,907 disclose manufacture of a large circuit on the surface of an aluminum plate. After fabrication of the circuit, and after microelectronic elements such as individual semiconductor chips are mounted to the circuit, a support ring is attached around the periphery of the circuit and the plate is removed. In this arrangement, the plate maintains dimensional stability of the circuit throughout the fabrication and mounting process. However, the additional process steps required to remove the plate considerably complicate use of this approach. Also, the plate precludes access to one side to the circuit which impedes the fabrication process and restricts the design of the finished product.
The aforementioned U.S. Pat. No. 5,518,964 and the corresponding PCT International Publication WO 96/02068, the disclosure of which is also hereby incorporated by reference herein, disclose processes in which circuit elements such as microelectronic connection components are fabricated in the form of a wafer-size sheet. In certain processes disclosed in the ""964 patent, a sheet of a starting material such as a flexible dielectric sheet with metallic layers thereon is stretched and bonded to a rigid frame having an opening or aperture therein so that the sheet is held taut by the rigid frame and maintained under tension by the frame. The frame may be in the form of a ring. The ring may be formed from a material such as molybdenum, which has a coefficient of thermal expansion close to that of a silicon semiconductor wafer, and lower than the coefficient of expansion of the sheet. The sheet may be stretched and attached as by bonding to the ring at an elevated temperature, so that the sheet remains in tension during processing at lower temperatures. While the sheet is held in the ring, it is accessible from both sides. The sheet is treated using various circuit-fabrication techniques such as etching and plating using photographically patterned resists. Because the sheet is maintained under tension throughout the process, it remains dimensionally stable. Because the sheet is accessible from both sides, fabrication of the sheet, and mounting of the sheet to the wafer can be performed readily. The features formed on the sheet are precisely positioned relative to one another over the entire extent of the sheet.
After processing, the entire sheet, with the rings still attached, can be aligned with a large assemblage of semiconductor chips such as a unitary semiconductor wafer. Leads formed during the fabrication process can be connected to all of the chips on the wafer. After connection, and after other processes such as deformation of leads on the sheet and injection of curable compliant material, the individual chips and associated portions of the sheet can be severed to provide individual packaged chips or subassemblies, each including one or more chips and an associated part of the sheet. Thus, the basic approach of using a rigid frame around the periphery of a sheet to provide dimensional stability during fabrication, as set forth in the ""964 patent, allows fabrication of microelectronic circuit elements in large arrays, such as wafer-size arrays with excellent dimensional stability and control. Further improvements in this basic approach are taught in the aforementioned commonly assigned U.S. patent application Ser. No. 08/690,532 filed Jul. 31, 1996, entitled Fixtures And Methods Of Lead Bonding And Deformation in certain preferred embodiments taught in the ""532 application, the sheet may be stretched by initially attaching it to a ring formed from a material of relatively high coefficient of thermal expansion such as aluminum at a low temperature such as room temperature, then heating the sheet and high-expansion ring and then attaching the sheet to a lower expansion ring such a molybdenum ring. As disclosed for example, in said U.S. Pat. No. 5,798,286 and in the corresponding PCT International Publication WO 97/11486, the disclosure of which is also hereby incorporated by reference herein, a frame-stretched sheet can be used in other assembly processes using individual semiconductor chips mounted individually to the sheet or mounted on a platen in a preselected array and bonded to the sheet as a unit.
Framed sheets have also been employed in unrelated arts and for different purposes. For example, thin framed sheets having referred to as pellicles are used in the optical arts as optical beam splitters. U.S. Pat. No. 4,037,111 discloses the use of a mechanically stretched sheet held taut by a borosilicate glass frame as a mask for X-ray lithography. German Offenlegungssachrift DE-3,919,564 A1 discloses fabrication of printed circuits by silk-screening onto a polyimide film held taut by an aluminum frame.
U.S. Pat. Nos. 3,537,169; 5,288,663; 5,169,804; 3,562,058 and 5,362,681 teach processes in which a wafer is adhered to a plastic film or xe2x80x9cdicing tape,xe2x80x9d then sawn into individual chips, whereupon the resulting chips are released from the film. In certain processes described in these patents, the film is carried by a frame.
The present invention includes aspects of the inventions disclosed in the aforementioned related applications.
One aspect of the present invention provides methods of making microelectronic components. A method according to this aspect of the invention includes the steps of providing a framed sheet including a flexible sheet and a frame having a structure defining an aperture, the sheet being secured to the structure of the frame so that the frame holds the sheet in tension. The method further includes the step of performing one or more operations on features on the flexible sheet which will ultimately be incorporated in the finished component. These operations are performed using one or more external elements other than the framed sheet itself. For example, the operations may include forming or treating the features on the framed sheet using external tools or engaging the features on the framed sheet with features of chips, wafers or other parts to be included in the finished microelectronic component. Methods according to this aspect of the invention most preferably include the step of registering the framed sheet with the external elements by aligning fiducial locations on the framed sheet with the external element.
In methods according to this aspect of the invention, the frame helps to maintain dimensional stability of the sheet as the sheet is registered and as the operations are performed. The enhanced dimensional stability of the sheet enables more precise registration of features on the sheet with external elements and with one another. Thus, distances between features of the sheet can be controlled with greater precision. For example, where the sequence of operations performed on the sheet includes multiple operations to form features, features formed in different operations can be aligned with one another more precisely. This is particularly useful where features formed in multiple operations must be linked to form continuous conductive features as, for example, where a lead formed in an etching or plating operation on one surface must join with a via, a terminal or lead formed in a separate operation. The more precise registration provided by the framed sheet processing technique permits fabrication of smaller features than could be fabricated in a comparable fabrication technique using an unframed sheet. Alternatively or additionally, the operations performed on the framed sheet may include bonding of the features on the sheet to other parts of the device such as, for example, bonding leads on the sheet to contacts of a chip or a wafer. Here again, the dimensional stability of the sheet provided by the film facilitates registration of the sheet features with the features of the external part.
The fiducial marks on the framed sheet may include features of the frame, and the registering step may include the step of mechanically engaging these features of the frame so as to control the position of the frame. Alternatively or additionally, the fiducial features of the framed sheet may include visible marks on the sheet or frame, which may be detected by manual or automated visual location. Preferably, the frame has a coefficient of thermal expansion different than the coefficient of thermal expansion of the sheet, in which case the tension in the sheet will vary with the temperature of the framed sheet.
A further aspect of the present invention provides methods of treating a sheet to form a microelectronic component. A method according to this aspect of the invention includes the step of bringing a flexible sheet and a frame having a structure defining an aperture to bonding temperatures and bonding the sheet to the structure of the frame while the frame and sheet are at such bonding temperatures so as to form a framed sheet. Preferably, both the frame and the sheet are at the same bonding temperature during the bonding operation. The method further includes the step of processing the framed sheet by performing operations on the features of the sheet which will be incorporated in the microelectronic component. At least a portion of this processing step is performed at a processing temperature or temperatures different from a bonding temperature used in the bonding operation. The sheet and the frame structure have different coefficients of thermal expansion such that as the framed sheet goes from the bonding temperature to the processing temperature, differential thermal expansion or contraction of the frame and sheet causes the frame to apply tension to the sheet. Desirably, the flexible sheet is held under tension during the bonding step, that the tension applied by differential thermal expansion or contraction will reinforce the tension applied during bonding. In a particularly preferred arrangement, the frame structure has a coefficient of thermal expansion lower than the coefficient of thermal expansion of the sheet and the bonding temperature is above the processing temperature. The coefficient of thermal expansion of the framed structure may, for example, be about equal to the coefficient of thermal expansion of silicon. Because the sheet expands and contracts with the frame, features on the sheet will remain substantially in registration with features on the silicon elements such as a wafer despite temperature changes encountered during processing.
Yet another aspect of the present invention provides a framed sheet for forming microelectronic components which includes a frame having a structure defining an aperture and a flexible sheet including a dielectric layer and a metallic portion adopted to form conductive features of a microelectronic component. The sheet extends across the aperture and is held in tension by the frame. The sheet has a coefficient of thermal expansion different from that of the frame so that as the frame and sheet vary in temperature, the tension in the sheet will also vary. Most preferably, the metallic portion of the sheet includes one or more full or partial layers of copper or a copper alloy and the frame structure has a coefficient of thermal expansion substantially lower than the coefficient of thermal expansion of the sheet. The framed sheets in accordance with this aspect of the invention can be utilized in the methods discussed above.